The present invention relates to a method of multi-address communication in a loop network which is formed by connecting a plurality of nodes (namely, communication control devices) each provided with a data processor, to one another through a transmission line, and more particularly to a method of multi-address communication capable of confirming the delivery of multicast information.
In a communication system where a plurality of data processors are connected to a bus through communication control devices (hereinafter referred to as "nodes"), to communicate with one another, two communication methods can be used. One of the methods is individual communication in which information is sent from a transmitting node to a specified receiving node, and the other method is multi-address communication in which the same information as transmitted from a transmitting node is received by all the nodes.
In the individual communication, each node is so constructed as to respond to the transmitted information when the node is specified as the receiving node, and the response can be made in two methods. In one of the methods, an information frame includes a response bit, and the receiving node sets the response bit to a state capable of indicating that the receiving node has succeeded in accepting the information frame. In the other method, the receiving node sends a response frame to the transmitting node when the receiving node receives an information frame transmitted from the transmitting node. The latter method is superior in the reliability of communication information than the former method.
The latter method is discussed in material paper No. 23 (1984), Working groups for Distributed Processing System in Information Processing Society of Japan, pp. 1-8.
FIG. 6 shows the construction of a conventional loop transmission system. In FIG. 6, reference characters a, b, c and d designate data processors, and reference numerals 2, 3, 4 and 5 designate nodes. As shown in FIG. 6, the nodes 2 to 5 are connected to one another through a transmission line 1, to form a loop network. In this network, a token passing access method is usually employed. FIG. 7 shows a conventional frame format. Referring to FIG. 7, a frame is bounded by delimiters 12 and 18, and includes a destination address field 13, a source address field 14, a frame command (or loop command) 15 for indicating the kind of the frame, an information field 16, and a frame check sequence field 17. Other frames than an information frame, that is, a token frame, a response frame and others do not include the information field 16. The response frame is classified into an accept (ACP) frame for indicating the success in accepting an information frame and a busy (BSY) frame for indicating the failure in accepting the information frame due to the busy state of a buffer which is to store the information frame.
FIG. 8 shows a response operation in individual communication which is performed when a receiving node receives an information frame. Referring to FIG. 8, when an information frame 7 transmitted from a node 2 which has acquired the transmitting right, is received by a node 4 whose address is recorded in the destination address field of the information frame 7, the node 4 transmits the ACP or BSY frame to the transmitting node 2. When a response period 30 set for response frame has elapsed, the transmitting node 2 transmits a token frame 9 to abandon the transmitting right. In the conventionl communication method, the response period is predetermined, and hence it is impossible that, in multi-address communication, all the nodes transmit response frames. Accordingly, in the conventional multi-address communication using the token ring system, as shown in FIG. 9, each node having received a multicast information frame 10 does not transmit any response frame. The above-mentioned material does not deal with the response method in multi-address communication.
As mentioned above, in a case where a node fails in accepting multicast information because of the busy state of a buffer for the multicast information, the conventional communication method cannot inform a transmitting node of the above failure.